It was sometime in the 1960's. My uncle was teaching in Bulgaria. He wanted to buy stuff in Britain, but wasn't allowed to take much money out of Bulgaria. My father was farming in Britain. He wanted to buy stuff in Bulgaria, but wasn't allowed to take much money out of Britain.
My uncle and father thought up a Cunning Plan. My father went to Bulgaria, where my uncle gave him money, and my father bought stuff. My uncle went to Britain, where my father gave him money, and my uncle bought stuff.
The Cunning Plan was possibly illegal? But it was Pareto Improving. My uncle preferred buying British stuff to buying Bulgarian stuff. My father preferred buying Bulgarian stuff to buying British stuff. The Bulgarian government wouldn't care whether it was my uncle or my father buying Bulgarian stuff. The British government wouldn't care whether it was my father or my uncle buying British stuff. (Even if they had hoarded money, instead of buying stuff, I can't see why either government would care whether it was my uncle or my father hoarding the money.)
The Cunning Plan was economically equivalent to barter. It was exactly as if my uncle had bought Bulgarian stuff, my father had bought British stuff, and then the two had swapped the stuff.
The Cunning Plan worked because my uncle and father knew each other, and trusted each other. What was needed was some sort of internet dating site, so that people like my uncle and my father could find each other, with the host checking that both sides could be trusted. But the dating site would need to be open to threesomes too, for cases where someone in Albania wants to buy stuff in Britain, someone in Britain wants to buy stuff in Chad Cyprus, and someone in Chad Cyprus wants to buy stuff in Albania. Or foursomes. Or orgies with all sorts of people from all sorts of countries taking part to greater or lesser extents.
What would happen if there were more farmers in Britain wanting to meet teachers in Bulgaria than vice versa? You would need some sort of price to adjust, so British farmers would get a worse deal, which would discourage some, and Bulgarian teachers would get a better deal, which would encourage some more.
If the British or Bulgarian governments tried to fix the price in that dating market, that would cause problems. There could be an excess demand or excess supply. Which would create all sorts of problems, and lead to rationing, because some British farmers or Bulgarian teachers wouldn't be able to find a partner. So the governments might need to decide who gets priority in finding a partner. And then people would start to think up even more Cunning Plans to get round those government decisions.
Or, we could forget all about exchange controls and Cunning Plans, and just have flexible exchange rates.